1. Field of the Invention
The present invention relates to an amplifier output stage, and in particular to current limiting circuitry for protecting the output stage against damage when the output terminal is shorted to ground.
2. Description of the Prior Art
In the output stages of operational amplifiers and other circuits, it is often necessary to provide a circuit which will limit the current in the output stage if the output is shorted to ground. If a current limit circuit is not provided, and the output is shorted to ground, the output stage will often supply several hundred milliamps of current. This can cause the entire circuit to be destroyed. For this reason, commercially available operational amplifiers typically have current limit circuits. These current limit circuits work well to limit the current, but also take away from the available voltage swing for the output stage.
The sacrifice of some output swing to accommodate current limit protection has not been a significant factor in many of the prior art operational amplifiers which use .+-.15 volt power supplies. Where lower supply voltages are desired (such as .+-.5 volts), however, the voltage drop across a resistor connected in series with the collector/emitter current path of the output transistors can significantly limit the amount of available output swing.